1. Field of the Invention
The present invention relates to a ferroelectric memory having a circuit for stably discharging pyroelectric charges generated in a ferroelectric capacitor without affecting write and read operations.
2. Description of the Related Art
It is well known that a ferroelectric material has a pyroelectric effect wherein electric charges are generated on the ferroelectric material by a change in temperature. The pyroelectric effect of ferroelectric material should be low to enabled desired memory characteristics in a ferroelectric memory. The amount of pyroelectric charge generated when the ferroelectric material is heated, is determined according to the heating rate. When there is no circuit through which pyroelectric charges are discharged, a back voltage is generated, thus switching the polarization. Therefore, a ferroelectric material having a low pyroelectric effect is required. A pyroelectric coefficient p is determined by the dielectric constant c and the amount of polarization Ps of the ferroelectric material as follows.
-p=.epsilon..beta.Ps
wherein, .beta. is a constant determined by the properties of the material. The pyroelectric coefficient, in general, has direction opposite to that of the polarization of the ferroelectric material since an atomic displacement direction during the polarization of the material is opposite to that during generation of the pyroelectric charges.
This can disturb the polarization characteristics of the ferroelectric material used to store information in a ferroelectric memory. For example, during the fabrication of a ferroelectric memory device, when the ferroelectric material generates pyroelectric charges in a certain thermal fluctuation environment, the charges may permanently affect the polarization, thus deteriorating the function of the memory.
FIG. 1a is a circuit diagram of a conventional ferroelectric random access memory unit cell. As shown in FIG. 1a, the unit cells of the conventional ferroelectric random access memory have a structure in which a ferroelectric capacitor 2 is connected to the source of a field effect transistor (FET) 1. Information is stored by generating a polarity in the ferroelectric material by a voltage applied to the ferroelectric capacitor 2 by the switching operation of the FET 1 and information is read by reading the voltage generated by the polarization of the ferroelectric capacitor 2. However, in the ferroelectric random access memory having the above structure, the ferroelectric material generates pyroelectric charges when information is stored or read, as shown in FIG. 1b. When there is no path through which the pyroelectric charges can be discharged, the pyroelectric charges are diffused to the source of the FET 1. The pyroelectric charges cannot be discharged because of the p-n junction source and thus accumulate. The accumulated pyroelectric charges generate a back voltage in the FET 1, thus switching the polarization and deteriorating the function of the memory.
A ferroelectric random access memory in which a path 3 for discharging pyroelectric charges generated in the ferroelectric capacitor 2 to ground has been provided in order to solve the pyroelectric charge problem, as shown in FIG. 2. A ferroelectric random access memory having a pyroelectric charge discharging path 3 has an advantage that a back voltage is formed by the pyroelectric charges. However, the path must be temporarily disrupted when information is stored in or read from the ferroelectric capacitor 2.